Sveins best friend is the hooded rat Halvorsen, everyone else thinks rats are nasty. Svein and his buddy Dan is quite alone until the new girl in class suggests they take their roots its in the big pet contest. The problems come when Halvorsen runs away, and the newspapers write that both the hospital and the school must close because of an invasion of rats. Svein must make sure that no mom and dad find out that it's Svein and Halvorsen, who is to blame for all the fuss, because he might end up losing both Halvorsen and the opportunity to win the competition.